ps3 terraria is in c++. the mobile versions are a port of that c++ code, and the 3ds version is a port of the mobile version. the ps3 version was discontinued, so there isn't really any upstream version of 1.4 to port. the modern mobile versions are made by DR Studios, and codeglue has been absorbed by behaviour interactive. which means there is no 1.4 c++ code to be able to port to 3ds(?). the new 3ds xl only has 256mb of ram, and uses an arm11 cpu for apps.
It is far more acheivable to game patch the existing version. the problem is that the existing version is limited due to terraria being a vanilla app with less control over the hardware. a homebrew version would need to be made, and therefore a direct port.
a 1.3 version for 3ds would be more acheivable as there already is a few 1.3 components in the code, and 1.3 doesn't use the fancy graphical effects. according to the terraria wiki dyes are not available on 3ds due to unavailability of pixel shaders. also the 3ds lacks things like target dummies and trapdoors.
if you tried to port a 1.4 version of c# terraria, it would require severe optimisations, possibly assembly tricks, and if someone was extreme using both arm9 and arm11 at the same time by using custom firmware to save resources by not using the OS (and instead booting terraria using firmware).
a 1.4 port would also be impractical due to the 3ds limited screen size/ resolution / controls. the controls are somewhat clunky already, the camera is zoomed far in, and the low resolution makes zooming out impractical as it would make the game unplayable, but things like the moonlord and celestial pillers are quite large, and bosses like duke fishron are already very difficult to fight due to the zoom. i doubt you'd be able to see the whole moon lord on screen at once, unless you sacrifice playability (or shrink moonlord).
